P2: Text disappears when checklist items in comments are edited #1981

Originally reported by @rachelmcr in https://github.com/Automattic/themes/issues/1981.

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## Steps to replicate

This report refers to the p2 checklist syntax (`o`) and occurs in posts/comments created with the classic or block editor. Example checklist:

```
o Item 1
o Item 2
o Item 3
```

1. In a comment on a p2 post, add a checklist (like the example above).
2. Click on one of the checklist items.
3. Select "Edit."
4. Notice that the checklist item text disappears.


## Result

When I add a checklist to a p2 post, editing a checklist item works as expected. The item's text is retained and I can make changes to it.

However, when I add a checklist to a p2 comment, editing the checklist item erases all the item's text content.

## Expected

I expect that when I add a checklist to a p2 post or comment, I will be able to click on each checklist item and edit the text.

Source: Dogfooding/testing (no external report)
Tested on macOS 10.15.4, Chrome 81.0.4044.129.

The issue names no files or tests. Start by reproducing the problem in a comment on a P2 post using the classic or block editor, then compare editing checklist items in posts and comments; done means the text remains available for editing in both contexts.

Written by the indexing model from the issue text.